Electrochemical fuel cells convert a fuel and an oxidant to electricity. Solid polymer electrochemical fuel cells generally employ an ion exchange membrane or some other kind of solid polymer electrolyte disposed between two electrodes, an anode and a cathode, each comprising a layer of catalyst to induce the desired electrochemical reaction. An embodiment of a conventional hydrogen fuel cell system is shown schematically at 10 in FIG. 1. It includes an anode 12 and a hydrogen gas inlet 14, and a cathode 18 and an air inlet 20. Hydrogen gas enters the fuel cell at the inlet 14 and is oxidized at anode 12 to form protons 16 and electrons 17. Oxygen, often from air, is reduced at cathode 18 to form water 22. The fuel cell system also includes a proton exchange membrane 24 for passage of protons from the anode 12 to the cathode 18. In addition to conducting hydrogen ions, the membrane 24 separates the hydrogen fuel stream from the oxidant stream. A conventional fuel cell also includes outlets 24 and 26 for oxidant and fuel, respectively.
In many conventional fuel cells, electrically conductive reactant flow field plates are used to direct pressurized reactant streams, which may be pressurized, to flow across the anode and cathode between the reactant stream inlet and outlet. Typically such reactant flow field plates have at least one flow passage or channel formed in one or both faces. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels or passages for the fuel and oxidant to the respective anode and cathode surfaces, and provide passages for the removal of reaction products, such as water, formed during operation of the cell.
Fuel cell performance can suffer significantly if there is not a sufficient supply of reactant to the entire electrode. Therefore, it has been a common practice in conventional fuel cells to provide excess reactants to the fuel cell in order to assure adequate supply at the electrode. In the case of the anode electrode, this generally wastes valuable fuel—reducing the fuel utilization, which is the ratio of the quantity of fuel supplied to the quantity of fuel actually consumed to produce electrical power. Ideally all of the fuel supplied to the fuel cell is used to produce power (a fuel utilization of 1 or 100%).
Some fuel cells are designed to operate in a closed mode on one or both of the reactant sides in an attempt to try to increase the reactant utilization. In these situations the reactant used on the closed side is generally substantially pure. Nonetheless, one of the problems associated with such systems is the accumulation of non-reactive components that tend to build up on the anode and dilute the local fuel concentration. If the fuel supply needed to support the power demand is not available (even locally within a particular fuel cell in the system), the fuel cell system may experience global or localized fuel starvation. Fuel starvation can cause permanent, irrecoverable, material damage to the fuel cells resulting in lower performance or eventual failure of the system.
There are various sources of the non-reactive components that tend to accumulate at the anode in a closed fuel system. One is impurities in the fuel stream itself—even if the fuel is substantially pure with a very low concentration of other components, these will tend to build up over time in a closed system. Also water produced at the cathode and nitrogen from the air (in air breathing configurations) will tend to cross the electrolyte and accumulate at the anode
A typical solution is the inclusion of a purge valve (which is normally closed in closed system operation) somewhere in the fuel passage for periodic venting of accumulations of non-reactive components, which can build up at the anode in closed system operation. In conventional fuel cell purge systems the purge valve is opened from time to time, for example, manually or at regular fixed time intervals, or in response to some monitored parameter. Alternatively, a continuous small vent of reactant may be used to prevent the accumulation of non-reactive components. The reactant flow path through the fuel cell system can be configured so that non-reactive components tend to accumulate first in just one or a few fuel cells of the fuel cell assembly, rather than in the outlet region of each cell in the assembly.
Such systems are not truly dead-ended, and although purging or a continuous vent can improve performance of fuel cells having closed fuel supply systems, it wastes valuable fuel—thereby reducing the fuel utilization. It also increases the parasitic load on the system and the complexity if purging equipment is required. Furthermore, the release of hydrogen into the ambient environment may be undesirable.